The present invention relates to hydraulic control apparatus for operating the support props of a mineral mining installation.
Known forms of control apparatus of the type with which the invention is concerned are described in German Patent Specifications P 27 49 312 and P 29 14 884. These apparatuses utilize manual-operable control valve devices which connect pressure fluid feed and return lines selectively to the working chambers of associated props. An automatic setting arrangement composed of further valve devices is designed to connect the prop chambers charged with fluid to extend the props to a pressure fluid source or to the pressure line once a threshold pressure is exceeded. The automatic setting arrangement acts independently of the control valve devices and ensures that a prop is adequately set against the roof even if one of the control valve devices is operated prematurely to disconnect the pressure source from the prop chamber. The threshold pressure at which the automatic setting arrangement comes into operation is made higher than that present in the prop chamber when the prop is being extended freely without contact with the roof and less than the desired setting pressure and that provided in the pressure feed line. In general, the props can be extended and retracted by operation of the control valve device(s) without the automatic setting arrangement coming into operation. This enables repairs and maintenance work and any back filling necessary in the event of roof falls to be performed without the automatic setting arrangement becoming affected. Generally, the threshold pressure at which the setting arrangement comes into operation is in the range 50 bars to 150 bars, e.g. 120 bars, while the pressure in the pressure feed line is significantly higher--typically above 300 bars and more usually in the range 350-450 bars.
One problem encountered with the known apparatuses, occurs when the props are subjected to pressure fluid to cause their retraction. During this operation pressure fluid in the working chambers of the props charged to produce their extension needs to pass back to the return line and it is possible that temporary fluctuations in the pressure in these chambers, caused by back pressure, could exceed the threshold pressure of the automatic setting system. In this event the retraction of the props will be prevented or hindered since the pressure source would be connected to the working chambers of the props instead of the return line.
A general object of the present invention is to provide an improved form of control apparatus.